Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Bond	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on available-for-sale securities, net of tax	$ 686,000	$ (719,000)
Available for sale securities remaining maturities greater than three years	0	0
Unrealized gain (loss) on available-for-sale securities, before tax	1,100,000	(700)	$ 0
Unrealized gain on available-for-sale securities, income tax expense	435,000	0	0
Aggregate fair value of securities in an unrealized loss position for less than twelve months	102,500,000	155,500,000
Other-than-temporary impaired	$ 0	0
Number of investment grade corporate bonds | Bond	34
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on available-for-sale securities, net of tax	$ 635,000
Common Equity Securities [Member] | Dimension Therapeutics, Inc. [Member]
Schedule Of Available For Sale Securities [Line Items]
Realized gains or losses available for sale securities		$ 0	$ 0
Common Equity Securities [Member] | Dimension Therapeutics, Inc. [Member] | Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Realized gains or losses available for sale securities	$ 100,000